High electron mobility transistor and method of forming the same

ABSTRACT

A semiconductor structure includes a first III-V compound layer. A second III-V compound layer is disposed on the first III-V compound layer and is different from the first III-V compound layer in composition. A carrier channel is located between the first III-V compound layer and the second III-V compound layer. A source feature and a drain feature are disposed on the second III-V compound layer. A gate electrode is disposed over the second III-V compound layer between the source feature and the drain feature. A fluorine region is embedded in the second III-V compound layer under the gate electrode. A diffusion barrier layer is disposed on top of the second III-V compound layer. A gate dielectric layer is disposed over the second III-V compound layer. The gate dielectric layer has a fluorine segment on the fluorine region and under at least a portion of the gate electrode.

PRIORITY CLAIM AND CROSS-REFERENCE

The present application is a continuation of U.S. application Ser. No. 13/774,614, entitled “High Electron Mobility Transistor and Method of Forming the Same,” filed on Feb. 22, 2013, which is incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates generally to a semiconductor structure and, more particularly, to a high electron mobility transistor (HEMT) and method for forming a high electron mobility transistor.

BACKGROUND

In semiconductor technology, due to their characteristics, Group III-Group V (or III-V) semiconductor compounds are used to form various integrated circuit devices, such as high power field-effect transistors, high frequency transistors, or high electron mobility transistors (HEMTs). A HEMT is a field effect transistor incorporating a junction between two materials with different band gaps (i.e., a heterojunction) as the channel instead of a doped region, as is generally the case for metal oxide semiconductor field effect transistors (MOSFETs). In contrast with MOSFETs, HEMTs have a number of attractive properties including high electron mobility, the ability to transmit signals at high frequencies, etc.

From an application point of view, enhancement-mode (E-mode) HEMTs have many advantages. E-mode HEMTs allow elimination of negative-polarity voltage supply, and, therefore, reduction of the circuit complexity and cost. Despite the attractive properties noted above, a number of challenges exist in connection with developing III-V semiconductor compound-based devices. Various techniques directed at configurations and materials of these III-V semiconductor compounds have been implemented to try and further improve transistor device performance.

Frequently, layers of a semiconductor are doped in the manufacturing process. Magnesium (Mg) is a common dopant for a P-type gallium nitride (p-GaN). Mg always diffuses into active layers and impacts performance, specifically in the 2-dimensional electron gas (2DEG) and current density of HEMT devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure may be understood from the following detailed description and the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 is a cross-sectional view of a semiconductor structure having a high electron mobility transistor (HEMT) according to one or more embodiments of this disclosure.

FIG. 2 is a flowchart of a method of forming a semiconductor structure having a HEMT according to one or more embodiments of this disclosure.

FIGS. 3 to 7 are cross-sectional views of a semiconductor structure having a HEMT at various stages of manufacture according to one embodiment of the method of FIG. 2.

DETAILED DESCRIPTION

The making and using of illustrative embodiments are discussed in detail below. It should be appreciated, however, that the disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative and do not limit the scope of the disclosure.

A plurality of semiconductor chip regions is marked on the substrate by scribe lines between the chip regions. The substrate will go through a variety of cleaning, layering, patterning, etching and doping steps to form integrated circuits. The term “substrate” herein generally refers to the bulk substrate on which various layers and device structures are formed. In some embodiments, the bulk substrate includes silicon or a compound semiconductor, such as GaAs, InP, SiGe, or SiC. Examples of such layers include dielectric layers, doped layers, polysilicon layers, diffusion barrier layers, or conductive layers. Examples of device structures include transistors, resistors, and/or capacitors, which may be interconnected through an interconnect layer to additional integrated circuits.

FIG. 1 is a cross-sectional view of a semiconductor structure 100 having a high electron mobility transistor (HEMT) and an InGaN diffusion barrier layer 130 according to one or more embodiments of this disclosure.

Referring to FIG. 1, the semiconductor structure 100 having a HEMT is illustrated. The semiconductor structure 100 includes a substrate 102. In some embodiments, the substrate 102 includes a silicon carbide (SiC) substrate, sapphire substrate or a silicon substrate.

The semiconductor structure 100 also includes a heterojunction formed between two different semiconductor material layers, such as material layers with different band gaps. For example, the semiconductor structure 100 includes a non-doped narrow-band gap channel layer and a wide-band gap n-type donor-supply layer. In at least one embodiment, the semiconductor structure 100 includes a first III-V compound layer (or referred to as a channel layer) 104 formed on the substrate 102 and a second III-V compound layer (or referred to as a donor-supply layer) 106 formed on the channel layer 104. The channel layer 104 and the donor-supply layer 106 are compounds made from the III-V groups in the periodic table of elements. However, the channel layer 104 and the donor-supply layer 106 are different from each other in composition. The channel layer 104 is undoped or unintentionally doped (UID). In the present example of the semiconductor structure 100, the channel layer 104 includes a gallium nitride (GaN) layer (also referred to as the GaN layer 104). The donor-supply layer 106 includes an aluminum gallium nitride (AlGaN) layer (also referred to as AlGaN layer 106). The GaN layer 104 and AlGaN layer 106 directly contact each other. In another example, the channel layer 104 includes a GaAs layer or InP layer. The donor-supply layer 106 includes an AlGaAs layer or an AlInP layer.

The GaN layer 104 is undoped. Alternatively, the GaN layer 104 is unintentionally doped, such as lightly doped with n-type dopants due to a precursor used to form the GaN layer 104. In one example, the GaN layer 104 has a thickness in a range from about 0.5 microns to about 10 microns.

The AlGaN layer 106 is unintentionally doped. In one example, the AlGaN layer 106 has a thickness in a range from about 5 nanometers (nm) to about 50 nm.

The InGaN diffusion barrier layer 130 may prevent the degradation of the HEMT device. The direction of the polarization-induced field formed in the InGaN is opposite to that in AlGaN thereby lifting the conduction band in the InGaN capped HEMTS. This design may decrease the 2DEG channel density and may create a positive Vth when compared to conventional AlGaN/GaN HEMTs.

The band gap discontinuity exists between the AlGaN layer 106 and the GaN layer 104. The electrons from a piezoelectric effect in the AlGaN layer 106 drop into the GaN layer 104, creating a very thin layer 108 of highly mobile conducting electrons in the GaN layer 104. This thin layer 108 is referred to as a two-dimensional electron gas (2DEG), forming a carrier channel (also referred to as the carrier channel 108). The thin layer 108 of 2DEG is located at an interface of the AlGaN layer 106 and the GaN layer 104. Thus, the carrier channel has high electron mobility because the GaN layer 104 is undoped or unintentionally doped, and the electrons can move freely without collision or with substantially reduced collisions with impurities.

The semiconductor structure 100 may also include an InGaN diffusion barrier layer 130 disposed between an AlGaN layer and a p-GaN layer. The InGaN diffusion barrier layer 130 may prevent diffusion into the HEMT active layer because the diffusion coefficient of Mg in InGaN is lower than in AlGaN. In one example, the InGaN diffusion barrier layer 130 has a thickness in a range from about 3 nm to about 15 nm.

The semiconductor structure 100 also includes a source feature and a drain feature disposed on the AlGaN layer 106 and configured to electrically connect to the carrier channel 108. Each of the source feature and the drain feature comprises a metal feature 112. In one example, the metal feature 112 is free of Au and comprises Al, Ti, or Cu.

The semiconductor structure 100 further includes a dielectric cap layer 110 disposed on a top surface of the AlGaN layer 106 not occupied by the metal features 112. The dielectric cap layer 110 further includes an opening that exposes a portion of the AlGaN layer 106 for a gate electrode formation, and a P-type GaN structure 123. The width of the opening in the dielectric cap layer 110 is greater than the width of the P-type GaN structure 123. The dielectric cap layer 110 protects the underlying AlGaN layer 106 from damage in the following process having plasma.

The semiconductor structure 100 further includes isolation regions 114 in the first III-V compound layer 104 and the second III-V compound layer 106. The isolation regions 114 isolate the HEMT in the structure 100 from other devices in the substrate 102. In one example, the isolation region 114 includes a doped region with species of oxygen or nitrogen.

The semiconductor structure 100 further includes a gate dielectric layer 119 deposited on the dielectric cap layer 110 and top surfaces of the source feature and the drain feature. The gate dielectric layer 119 is also disposed along an interior surface of the opening and on the exposed portion of the AlGaN layer 106. In one example, the gate dielectric layer 119 has a thickness in a range from about 3 nm to about 20 nm. In some examples, the gate dielectric layer 119 comprises silicon oxide, silicon nitride, gallium oxide, aluminum oxide, scandium oxide, zirconium oxide, lanthanum oxide or hafnium oxide.

In some embodiments, the semiconductor structure 100 further includes a protection layer (not shown). The protection layer is disposed on top surfaces of the metal features 112 and under the gate dielectric layer 119. The protection layer further includes an opening that aligns with the opening in the dielectric cap layer 110. The combined opening of the opening in the protection layer and the opening in the dielectric cap layer 110 exposes the portion of the AlGaN layer 106 and a P-type GaN structure 123 for the gate electrode formation. The protection layer also covers the source feature and the drain feature, and prevents the source feature and the drain feature from exposure during an annealing process in the formation of the isolation regions 116.

The semiconductor structure 100 also includes a gate electrode 128 disposed on the opening over AlGaN layer 106 between the source and drain features. The gate electrode 128 includes a conductive material layer configured for voltage bias and electrical coupling with the carrier channel 108. In various examples, the conductive material layer includes a refractory metal or its compounds, e.g., titanium (Ti), titanium nitride (TiN), titanium tungsten (TiW) and tungsten (W). In another example, the conductive material layer includes nickel (Ni), gold (Au) or copper (Cu).

The semiconductor structure 100 also includes a depletion region 126 in the carrier channel 108 under the opening in the dielectric cap layer 110. The carrier channel 108 becomes normally-off because of the depletion region 126. A positive gate voltage should be applied to turn on the carrier channel 108 of this HEMT. This HMET is also called an enhanced-mode HEMT that is opposite to a depletion-mode HEMT. The depletion-mode HEMT has a normally-on carrier channel and a negative gate voltage should be applied to turn off the carrier channel.

In the above described embodiments, the gate electrode 128, the source/drain features, and the carrier channel 108 in the GaN layer 104 are configured as a transistor. When a voltage is applied to the gate stack, a device current of the transistor could be modulated.

FIG. 2 is a flowchart of a method 200 of forming a semiconductor structure having a HEMT according to one or more embodiments of this disclosure. Referring now to FIG. 2, the flowchart of the method 200, at operation 201, a first III-V compound layer is provided. The first III-V compound layer is formed on a substrate. Next, the method 200 continues with operation 202 in which a second III-V compound layer is epitaxially grown on the first III-V compound layer. The method 200 continues with operation 203 in which a source feature and a drain feature are formed on the second III-V compound layer. The method 200 continues with operation 204 in which a gate dielectric layer is deposited on a portion of the second III-V compound layer. The method 200 continues with operation 206 in which a gate electrode is formed on the treated gate dielectric layer between the source feature and the drain feature. It should be noted that additional processes may be provided before, during, or after the method 200 of FIG. 2.

FIGS. 3 to 7 are cross-sectional views of the semiconductor structure 100 having a HEMT at various stages of manufacture according to various embodiments of the method 200 of FIG. 2. Various figures have been simplified for a better understanding of the inventive concepts of the present disclosure.

Referring to FIG. 3, which is an enlarged cross-sectional view of a portion of a substrate 102 of a semiconductor structure 100 after performing operations 201, 202 and 203. In some embodiments, the substrate 102 includes a silicon carbide (SiC) substrate, sapphire substrate or a silicon substrate. A first III-V compound layer 104, also referred to as a channel layer, is grown on the substrate 102. In the embodiment of FIGS. 2-7, the first III-V compound layer 104 refers to a gallium nitride (GaN) layer (also referred to as the GaN layer 104). The GaN layer 104 can be epitaxially grown by metal organic vapor phase epitaxy (MOVPE) using gallium-containing precursor and nitrogen-containing precursor. The gallium-containing precursor includes trimethylgallium (TMG), triethylgallium (TEG), or other suitable chemical. The nitrogen-containing precursor includes ammonia (NH₃), tertiarybutylamine (TBAm), phenyl hydrazine, or other suitable chemical. In the embodiment of FIGS. 2-7, the GaN layer 104 has a thickness in a range from about 0.5 micron to about 10 microns. In other embodiments, the first III-V compound layer 104 may include a GaAs layer or InP layer.

A second III-V compound layer 106, also referred to as donor-supply layer, is grown on first III-V compound layer 104. An interface is defined between the first III-V compound layer 104 and the second III-V compound layer 106. A carrier channel 108 of 2DEG is located at the interface. In at least one embodiment, the second III-V compound layer 106 refers to an aluminum gallium nitride (AlGaN) layer (also referred to as the AlGaN layer 106). In the embodiment of FIGS. 2-7, the AlGaN layer 106 is epitaxially grown on the GaN layer 104 by MOVPE using aluminum-containing precursor, gallium-containing precursor, and nitrogen-containing precursor. The aluminum-containing precursor includes trimethylaluminum (TMA), triethylaluminium (TEA), or other suitable chemical. The gallium-containing precursor includes TMG, TEG, or other suitable chemical. The nitrogen-containing precursor includes ammonia, TBAm, phenyl hydrazine, or other suitable chemical. In the embodiment of FIGS. 2-7, the AlGaN layer 106 has a thickness in a range from about 5 nanometers to about 50 nanometers. In other embodiments, the second III-V compound layer 106 may include an AlGaAs layer, or AlInP layer.

Further, the second III-V compound layer 106 may include an InGaN diffusion barrier layer 130. The InGaN diffusion barrier layer 130 may be grown at a range of about 300 mbar to about 500 mbar and in a range of about 700° C. to about 900° C. The Indium composition of the InGaN diffusion barrier layer 130 may be in a range from about 5% to about 10%.

Thereafter, a P-type GaN layer is disposed onto the second III-V compound layer 106. A patterned mask layer (i.e., a photoresistive layer not shown) is formed on the top surface of the P-type GaN layer and an etching process is performed to remove a portion of the P-type GaN layer. As a result of this lithographic process, a P-type GaN structure 123 is formed.

Next, a dielectric cap layer 110 is deposited on a top surface 107 of the second III-V compound layer 106 and over the top surface of the P-type GaN structure 123. Therefore, the P-type GaN structure 123 is effectively embedded in the dielectric cap layer 110. In the embodiment of FIGS. 2-7, the dielectric cap layer 110 has a thickness in a range from about 100 Å to about 5000 Å. In some embodiments, the dielectric cap layer 110 includes SiO₂ or Si₃N₄. In one example, the dielectric cap layer 110 is Si₃N₄ and is formed by performing a low pressure chemical vapor deposition (LPCVD) method without plasma using SiH₄ and NH₃ gases. An operation temperature is in a range of from about 650° C. to about 800° C. An operation pressure is in a range of about 0.1 Torr and about 1 Torr. The dielectric cap layer 110 protects the underlying second III-V compound layer 106 from damage in the following processes having plasma. Next, two openings in the dielectric cap layer 110 are defined by lithography and etching processes to expose a portion of the second III-V compound layer 106.

A metal layer is deposited over the dielectric cap layer 110, overfills the openings and contacts the second III-V compound layer 106. A photoresist layer (not shown) is formed over the metal layer and developed to form a feature over the openings. The metal layer not covered by the feature of the photoresist layer is removed by a reactive ion etch (RIE) process that etches the exposed portions of the metal layer down to the underlying the dielectric cap layer 110. Metal features 112 are generated after the etching process. The metal features 112 are configured as the source feature or the drain feature for the HEMT. The photoresist layer is removed after the formation of the metal features 112. The dielectric cap layer 110 protects the underlying second III-V compound layer 106 from damage during the etching process to form metal features 112. The carriers in carrier channel 108 of 2DEG underlying the second III-V compound layer 106 would not be affected during the etching process. The electrical performances of the semiconductor structure 100 would be positively affected. Therefore, the yield of the overall assembly could increase.

In some embodiments, the metal layer of the metal features 112 includes one or more conductive materials. In at least one example, the metal layer is free of gold (Au) and comprises titanium (Ti), titanium nitride (TiN), or aluminum copper (AlCu) alloy. In another example, the metal layer includes a bottom Ti/TiN layer, an AlCu layer overlying the bottom Ti/TiN layer, and a top Ti layer overlying the AlCu layer. The formation methods of the metal layer include atomic layer deposition (ALD) or physical vapor deposition (PVD) processes. Without using Au in the metal features 112, the method 200 could also be implemented in the production line of integrated circuits on silicon substrate. The contamination concern from Au on the silicon fabrication process could be eliminated.

Next, a protection layer (not shown) is optionally deposited on top surfaces of the metal features 112 and the dielectric cap layer 110. In some embodiments, the protection layer includes dielectric materials such as SiO₂ or Si₃N₄. In one example, the protection layer is Si₃N₄ and is formed by performing a plasma enhanced chemical vapor deposition (PECVD) method.

FIG. 4 illustrates the structure 100 after forming isolation regions 114 in the first III-V compound layer 104 and the second III-V compound layer 106. The isolation regions 114 isolate the HEMT in the structure 100 from other devices in the substrate 102. In one example, the isolation region 114 is formed by an implantation process with species of oxygen or nitrogen. The protection layer covers the source feature and the drain feature, and prevents the source feature and the drain feature from exposure during an annealing process after the implantation process for the isolation region 114 formation.

FIG. 5 illustrates the structure 100 after forming an opening 116 in the dielectric cap layer 110 (also in the protection layer if the protection layer exists). A patterned mask layer (not shown) is formed on a top surface of the dielectric cap layer 110 and an etching process is performed to remove a portion of the dielectric cap layer 110 (also remove a portion of the protection layer if the protection layer exists). The opening 116 exposes a portion of the top surface 107 of the second III-V compound layer 106, and exposes the top surface of P-type GaN structure 123. In particular, the width of the opening 116 is greater than the width of the P-type GaN structure 123, so that some portion of the second III-V compound layer 106 is exposed on either side of the P-type GaN structure 123. The opening 116 is configured as a location for the later gate electrode formation.

FIG. 6 illustrates the structure 100 after depositing a gate dielectric layer 118 in operation 204. The gate dielectric layer 118 is deposited on the dielectric cap layer 110, along an interior surface of the opening 116 and on the exposed portion of the second III-V compound layer 106, and the top surface of the P-type GaN structure 122. The gate dielectric layer 118 is also deposited over the source feature and the drain feature. In some embodiments, the gate dielectric layer 118 is in a thickness range from about 3 nm to about 20 nm. In some examples, the gate dielectric layer 118 comprises silicon oxide, silicon nitride, gallium oxide, aluminum oxide, scandium oxide, zirconium oxide, lanthanum oxide or hafnium oxide. In one embodiment, the gate dielectric layer 118 is formed by an atomic layer deposition (ALD) method. The ALD method is based on the sequential use of a gas phase chemical process. The majority of ALD reactions use two chemicals, typically called precursors. These precursors react with a surface one-at-a-time in a sequential manner. By exposing the precursors to the growth surface repeatedly, the gate dielectric layer 118 is deposited. The ALD method provides an uniform thickness of the gate dielectric layer 118 with high quality. In one example, the gate dielectric layer 118 is zirconium oxide. In some embodiments, a first precursor includes tetrakis[ethylmethylamino]zirconium (TEMAZr) or zirconium chloride (ZrCl₄). In some embodiments, a second precursor includes oxygen in order to oxidize the first precursor material to form a monolayer. In some examples, the second precursor includes ozone (O₃), oxygen, water (H₂O), N₂O or H₂O—H₂O₂. In other embodiments, the gate dielectric layer 118 is formed by a plasma enhanced chemical vapor deposition (PECVD) or a low pressure chemical vapor deposition (LPCVD).

FIG. 7 illustrates the structure 100 after performing operation 206, which forms a gate electrode 128 on the P-type GaN structure 123 of the gate dielectric layer 119. In one example, a gate electrode layer is deposited on the gate dielectric layer 119 and overfills the opening 116 shown in FIG. 6. Lithography and etching processes are performed on the gate electrode layer to define the gate electrode 128 between the source and drain features. In some embodiments, the gate electrode 128 includes a conductive material layer that includes a refractory metal or its compounds, e.g., titanium (Ti), titanium nitride (TiN), titanium tungsten (TiW) and tungsten (W). In another example, the gate electrode 128 includes nickel (Ni), gold (Au) or copper (Cu).

Various embodiments of the present disclosure may be used to improve the performance of a semiconductor structure having a high electron mobility transistor (HEMT). For example, in conventional methods, a portion of the second III-V compound layer 106 is etched to form a recess for an enhanced-mode HEMT. During etching the recess, the etching uniformity among the semiconductor chip regions on the same substrate 102 is hard to control. The electrical performances of each HEMT in the same semiconductor chip region or the same substrate 102 could not be accurately controlled. In this disclosure, the fluorine region 124 depletes the electrons in the carrier channel 108 for an enhanced-mode HEMT. The fluorine region 124 in the opening 116 eliminates the drawbacks in conventional methods. The fluorine-treated gate dielectric layer 119 also improves stability of the threshold voltage (Vt) of the HEMT in the structure 100. The metal feature 112 is free of Au and comprises Al, Ti or Cu. Without using Au in the metal feature 112, the method 200 can be implemented in the production line of integrated circuits on silicon substrate, because the contamination concern from Au on the silicon-Fab process is eliminated. Compared with the HEMT with Au in source/drain feature, the cost for manufacturing the HEMT according to the present application is reduced. Both the III-V semiconductor compounds process and the silicon-fabrication process can be implemented in the same production line. It increases the flexibility to allocate different products for the production line.

One aspect of the disclosure describes a semiconductor structure. The semiconductor structure includes a first III-V compound layer. A second III-V compound layer is disposed on the first III-V compound layer and different from the first III-V compound layer in composition. A carrier channel is located between the first III-V compound layer and the second III-V compound layer. A source feature and a drain feature are disposed on the second III-V compound layer. A gate electrode is disposed over the second III-V compound layer between the source feature and the drain feature. A fluorine region is embedded in the second III-V compound layer under the gate electrode. A gate dielectric layer is disposed over the second III-V compound layer. The gate dielectric layer has a fluorine segment on the fluorine region and under at least a portion of the gate electrode.

A further aspect of the disclosure describes a semiconductor structure. The semiconductor structure includes a GaN layer disposed on a substrate. An AlGaN layer is disposed on the GaN layer. A source feature and a drain feature spaced apart are disposed on the AlGaN layer. A gate electrode is disposed over the AlGaN layer between the source feature and the drain feature. A portion of a gate dielectric layer is disposed between the gate electrode and the AlGaN layer. An InGaN diffusion barrier layer 130 may be disposed on top of the AlGaN layer and beneath an Magnesium doped p-GaN cap layer.

The present disclosure also describes an aspect of a method of forming a semiconductor structure. The method includes providing a first III-V compound layer. A second III-V compound layer is epitaxially grown on the first III-V compound layer. A carrier channel is located between the first III-V compound layer and the second III-V compound layer. A source feature and a drain feature are formed on the second III-V compound layer. A gate dielectric layer is deposited on a portion of the second III-V compound layer. The gate dielectric layer on the portion of the second III-V compound layer is treated with fluorine. A gate electrode is formed on the treated gate dielectric layer between the source feature and the drain feature.

According to an embodiment, a semiconductor structure comprises a first III-V compound layer, and a second III-V compound layer disposed on the first III-V compound layer and different from the first III-V compound layer in composition. The semiconductor structure further comprises a third III-V compound layer disposed over the second III-V compound layer, wherein a diffusion barrier layer is interposed between the second III-V compound layer and the third III-V compound layer, and a source contact and a drain contact disposed on the second III-V compound layer, wherein the third III-V compound layer is interposed between the source contact and the drain contact.

According to another embodiment, a semiconductor structure comprises a first III-V material, and a second III-V material disposed on the first III-V material, the first III-V material and the second III-V material forming an interface therebetween. The semiconductor structure further comprises a source contact and a drain contact disposed on the second III-V material, a gate electrode disposed over the second III-V material between the source contact and the drain contact, and a diffusion barrier layer on a portion of the second III-V material. The semiconductor structure further comprises a third III-V material disposed on the diffusion barrier layer, and a gate dielectric layer disposed over portions of the second III-V material and over a top surface of the third III-V material.

According to yet another embodiment, a method of forming a semiconductor structure, the method comprises forming a second III-V material on a first III-V material, wherein the second III-V material is different from the first III-V material, forming a source contact and a drain contact on the second III-V material, and forming a diffusion barrier layer on the second III-V material. The method further comprises forming a third III-V material on the diffusion barrier layer, depositing a gate dielectric layer on a portion of the second III-V material and a top surface of the third III-V material, and forming a gate electrode between the source contact and the drain contact.

Although the embodiments and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed is:
 1. A semiconductor structure comprising: a first III-V compound layer; a second III-V compound layer disposed on the first III-V compound layer and different from the first III-V compound layer in composition; a third III-V compound layer disposed over the second III-V compound layer, wherein a diffusion barrier layer is interposed between the second III-V compound layer and the third III-V compound layer; and a source contact and a drain contact disposed on the second III-V compound layer, wherein the third III-V compound layer and the diffusion barrier layer are interposed between the source contact and the drain contact.
 2. The semiconductor structure of claim 1, further comprising a gate electrode disposed over the third III-V compound layer.
 3. The semiconductor structure of claim 2, wherein a width of the third III-V compound layer is less than a width of the gate electrode.
 4. The semiconductor structure of claim 2, further comprising a gate dielectric layer, the gate dielectric layer being interposed between the gate electrode and the second III-V compound layer.
 5. The semiconductor structure of claim 1, wherein a channel is located at an interface between the first III-V compound layer and the second III-V compound layer.
 6. The semiconductor structure of claim 5, wherein the channel is a two-dimensional electron gas.
 7. The semiconductor structure of claim 1, further comprising a dielectric cap layer disposed over the second III-V compound layer, the source contact and the drain contact extending through the dielectric cap layer.
 8. A semiconductor structure comprising: a first III-V material; a second III-V material disposed on the first III-V material, the first III-V material and the second III-V material forming an interface therebetween; a source contact and a drain contact disposed on the second III-V material; a gate electrode disposed over the second III-V material between the source contact and the drain contact; a diffusion barrier layer on a portion of the second III-V material; a third III-V material disposed on the diffusion barrier layer; and a gate dielectric layer disposed over portions of the second III-V material and over a top surface of the third III-V material, wherein the gate dielectric layer is disposed over the source contact and the drain contact.
 9. The semiconductor structure of claim 8, further comprising a dielectric cap layer, the dielectric cap layer being interposed between the gate dielectric layer and the second III-V material.
 10. The semiconductor structure of claim 9, wherein the dielectric cap layer has a thickness between about 100 Å and about 5000 Å.
 11. The semiconductor structure of claim 8, wherein a channel is located at the interface between the first III-V material and the second III-V material.
 12. The semiconductor structure of claim 11, wherein the channel is a two-dimensional electron gas.
 13. The semiconductor structure of claim 8, wherein a width of the diffusion barrier layer is less than a width of the gate electrode.
 14. The semiconductor structure of claim 8, wherein a width of the diffusion barrier layer is less than a width of the second III-V material.
 15. A method of forming a semiconductor structure, the method comprising: forming a second III-V material on a first III-V material, wherein the second III-V material is different from the first III-V material; forming a source contact and a drain contact on the second III-V material; forming a diffusion barrier layer on the second III-V material; forming a third III-V material on the diffusion barrier layer; depositing a gate dielectric layer on a portion of the second III-V material and a top surface of the third III-V material; and forming a gate electrode between the source contact and the drain contact, at least a portion of the gate electrode extending along a sidewall of the third III-V material.
 16. The method of claim 15, wherein the forming the third III-V material comprises doping the third III-V material.
 17. The method of claim 15, further comprising forming a dielectric cap layer on the second III-V material, the diffusion barrier layer being disposed in an opening in the dielectric cap layer.
 18. The method of claim 15, wherein a width of the diffusion barrier layer is substantially equal to a width of the third III-V material.
 19. The method of claim 15, wherein the forming the second III-V material comprises epitaxially growing the second III-V material on the first III-V material.
 20. The method of claim 15, wherein a two-dimensional electron gas is formed at an interface between the first material III-V material and the second III-V material. 